Method of making a cemented carbide powder with low sintering shrinkage and the powder obtained

ABSTRACT

A method of making cemented carbide powder with low sintering shrinkage including WC and 4-15 wt-% Co and up to 20 wt-% cubic carbide forming elements from the Groups 4 and 5 of the Periodic Table of the Elements by the powder metallurgical techniques wet milling, pressing and sintering. According to the method wet milling is performed in a rotating ball mill with a ratio between the weight of milling bodies and powder of 2-5. The milling bodies are shaped either as spheres or cylinders with semi-spherical end surfaces. The spherical bodies have a diameter of 10 to 15 mm and the cylindrical bodies have a diameter and height of 10 to 15 mm. The composition of the milling bodies is WC with 6 to 10 wt-% Co. A powder made according to the method is described.

The present invention relates to a method of making cemented carbide powder with low sintering shrinkage, particularly useful for cutting tool inserts for turning, milling and drilling of metals and the powder obtained.

Cemented carbide bodies is made by wet milling of powders forming hard constituents and binder phase to a slurry, drying the slurry generally by spray drying, pressing the dried powder in pressing tools to bodies of desired shape and finally sintering.

The milling operation is an intensive milling performed in mills with milling bodies. There are essentially two types of mills: rotating ball mills and attritor mills. In a rotating ball mill, the milling time is conventionally within 15 to 60 hours and such processing is believed to be necessary in order to obtain a uniform distribution of the constituents.

During sintering, the bodies shrink about 16-20% linearly. The shrinkage depends on pressing pressure, WC grain size, grain size distribution and Co-content. Pressing tools are expensive to make and are therefore made for a standard shrinkage such as 17.5%. The shrinkage is determined at a standard pressing pressure. If the shrinkage at the standard pressure is high, the pressing pressure at the predetermined shrinkage will be high. If the shrinkage at the standard pressure is low, the pressing pressure at the predetermined shrinkage will be low. A high pressing pressure is not desirable because of the risk of pressing cracks in the pressed bodies and abnormal wear and even risk of pressing tool failure including injuries to humans. A low pressing pressure may lead to bodies that are not fully dense after sintering. Moreover, dimensional control of the sintered bodies is facilitated if the pressing pressure is kept within a certain interval. Inserts produced with a high pressing pressure often show edges with pressing cracks.

The milling bodies can be in various forms as balls or short cylindrical rods. U.S. Pat. No. 3,531,280 discloses the size to be about ⅛ to ¼ inch and the material of the milling bodies to be cobalt bonded tungsten carbide containing about 6% cobalt. U.S. Pat. No. 3,525,610 discloses the size of the milling bodies to be within 0.1 to 0.3 inch and the material to be WC cemented with 5 to 10 wt-% Co. This size interval is chosen in order to obtain milling bodies with sufficient weight and size to obtain a high milling efficiency.

EP 1043413 discloses a method of making a cemented carbide with submicron WC grain size with a low compacting pressure. The method consists in premixing all components except WC for about three hours, adding the WC powder and then finally milling for about ten hours.

EP 1749601 discloses a method of making a ready to press cemented carbide powder with low compaction pressure suitable for the production of submicron cemented carbide. The method comprises using 1-3 wt-% pressing agent of the following composition, <90 wt-% PEG and 10-75 wt-% of blends of high molecular weight (C12-<C20) saturated or unsaturated fatty acids, or salts thereof containing at least one element of Al, Ba, Ca, Co, Cr, Mg, N, Na, V or Zn.

It has now surprisingly been found that a cemented carbide powder with a low sintering shrinkage at a constant pressing pressure can be made from powder mixtures that are produced in ball mills with milling bodies larger than conventional.

The present invention relates to a method of making a cemented carbide powder with low sintering shrinkage and excellent compacting properties for cemented carbide bodies preferably cutting tool inserts for metal machining comprising WC and 4-15 wt-% Co, preferably 5-12 wt-% Co and up to 20 wt-% cubic carbide forming elements from the Groups 4b and 5b of the Periodic Table of the Elements preferably Ti, Zr, Ta and Nb by means of the powder metallurgical techniques wet milling, pressing and sintering.

The wet milling is performed in a rotating ball mill with a ratio between the weight of milling bodies and powder of 2-5. The milling bodies are shaped either as spheres or cylinders with semi-spherical end surfaces. The spherical bodies have a diameter of 10 to 15 mm, preferably 11 to 14 mm. The cylindrical bodies have a diameter and height of 10 to 15 mm, preferably 11 to 14 mm. The composition of the milling bodies is WC with 6 to 10 wt-% Co, preferably 7 to 9 wt-% Co with a sintered WC grain size of 1 to 5 μm. In order to reach the desired grain size of the milled powder it may be necessary to prolong the milling time compared to milling according to prior art.

According to the invention there is also provided a cemented carbide powder with desired low sintering shrinkage and excellent compacting properties for cemented carbide bodies preferably cutting tool inserts for metal machining comprising WC and 4-15 wt-% Co, preferably 5-12 wt-% Co and up to 20 wt-% cubic carbide forming elements from the Groups 4b and 5b of the Periodic Table of the Elements preferably Ti, Zr, Ta and Nb.

The WC-grains have an FSSS average grain size in the range 3-9 μm, preferably 4-7 μm. The powder has a sintering shrinkage of 16.5 to 17.5%, preferably within 16.7 to 17.3% at a compacting pressure of 123 MPa.

EXAMPLE 1

A cemented carbide powder with the composition WC-9 wt-% Co, 0.5 wt-% (Ta, Nb)C (90/10) and 2.0 wt-% PEG 3400 was prepared according to the invention. The Fisher Sub Sieve Sizer (FSSS) value of the WC was 5.4 μm. The total batch weight was 450 kg and the weight of the milling bodies was 1340 kg. The milling was carried out in a mixture of 93.2 litres ethanol and 20 wt-% water. The milling bodies were cylindrical with diameter and height of 12 mm and a composition of WC with 8.5 wt-% Co and grain size 2.5 μm. The milling time was 32 hours. The powder batch was spray dried and compacted to bodies with the approximate size 15×15×7 mm at the standard pressure 123 MPa. The bodies were sintered at 1430° C. at standard sintering conditions. The exact dimensions of the bodies were determined before and after sintering.

The linear shrinkage of the bodies was 17.0% and the coercivity was 9.3 kA/m indicating an average grain size of 2.4 μm. Metallographic investigation indicated a porosity of A00B00C00.

EXAMPLE 2

Example 1 was repeated with exception of the milling time, which was 16 hours.

The linear shrinkage of the bodies was 16.9% and the coercivity was 8.4 kA/m indicating an average grain size of 2.8 μm. Metallographic investigation indicated a porosity of A00B00C00.

EXAMPLE 3 Prior Art

Example 1 was repeated with exception of the size of the milling bodies that was 8 mm.

The linear shrinkage of the bodies was 18.1% and the coercivity was 9.9 kA/m indicating an average grain size of 2.2 μm. Metallographic investigation indicated a porosity of A00B00C00.

EXAMPLE 4 Prior Art

Example 2 was repeated with exception of the size of the milling bodies that was 8 mm.

The linear shrinkage of the bodies was 17.8% and the coercivity was 9.1 kA/m indicating an average grain size of 2.5 μm. Metallographic investigation indicated a porosity of A00B00C00.

Thus, the Examples 1-4 show that milling with large milling bodies leads to a powder with a low sintering shrinkage implying that the compacting pressure will be low at a given shrinkage. Furthermore, the Examples show that a longer milling time is needed in order to obtain the same coercivity/grain size of the sintered body. However, compacting at a low pressure is preferred to milling for a long time. 

1. Method of making cemented carbide powder with low sintering shrinkage comprising WC and 4-15 wt-% Co and up to 20 wt-% cubic carbide forming elements from the Groups 4b and 5b of the Periodic Table of the Elements by means of the powder metallurgical techniques wet milling, pressing and sintering characterised in wet milling in a rotating ball mill with a ratio between the weight of milling bodies and powder of 2-5, the milling bodies being shaped either as spheres or cylinders with semi-spherical end surfaces whereby the spherical bodies have a diameter of 10 to 15 mm and the cylindrical bodies have a diameter and height of 10 to 15 mm and with a composition of the milling bodies of WC with 6 to 10 wt-% Co.
 2. Cemented carbide powder with low sintering shrinkage comprising WC and 4-15 wt-% Co and up to 20 wt-% cubic carbide forming elements from the Groups 4b and 5b of the Periodic Table of the Elements with WC-grains with an average grain size in the range 3-9 μm characterised in that the powder has a sintering shrinkage of 16.5 to 17.5%, preferably of 16.7 to 17.3% at a compacting pressure of 123 MPa. 